This invention relates to a driving wheel slip control system for an automotive vehicle, and more particularly to a system of this kind in which the output of an engine in the vehicle is decreased when an excessive slip of a driving wheel thereof is sensed. Further, the invention relates to improvements in or to such systems, which enable proper slip control when the driving wheels have shifted to an excessive slip state, and when they have left the excessive slip state.
As recognized in general, a driving wheel of an automotive vehicle undergoes a slip when the vehicle starts to move or when it under acceleration, if the driving force of the driving wheel surpasses a frictional force developed between the tire of the driving wheel and the road surface [=the coefficient of friction between the tire and the road surface = load of the vehicle weight on the driving wheel (wheel load)]. The frictional force acts in the advancing or longitudinal direction of the vehicle. The magnitude of the slip may be represented by a slip rate .lambda. which is expressed by the following equation: EQU .lambda.=(V.sub.W- V)/V.sub.W (I)
where V.sub.W represents the circumferential velocity of the driving wheel, and V represents the speed of the vehicle.
The frictional force F between the driving wheel tire and the road surface, which defines the upper limit of the effective driving force of the driving wheel, varies with the slip rate .lambda., as shown in FIG. 1. It will be noted from the figure that the frictional force F assumes the maximum value when the slip rate .lambda., has a predetermined value .lambda..sub.0. While the longitudinal frictional force F varies with slip rate .lambda. as indicated by the solid line in the graph, the transverse frictional force, which acts in the transverse direction of the vehicle, varies with slip rate .lambda., as indicated by the broken line in the graph, such that it becomes smaller as the slip rate increases.
A slip prevention system has been proposed, e.g. by Japanese Patent Publication (Kokoku) No. 52-35837, which is based upon the above recognition of the relationship between longitudinal frictional force, transverse (or lateral) frictional force and slip rate, and which controls the slip rate such that the longitudinal frictional force may be the maximum so as to obtain the maximum driving efficiency of the vehicle, while the transverse frictional force may have a drop as small as possible so as to prevent a skid or sideslip of the vehicle.
The above proposed control system is constructed such that the slip of the driving wheels is prevented from becoming excessive through control of the torque of the engine by turning on and off an ignition device of the engine of the vehicle or by allowing and inhibiting fuel supply from a fuel supply device to the engine. However, according to the proposed control system, immediately when either one of the driving wheels has shifted from a moderate slip state to an excessive slip state, fuel supply is suddenly interrupted, for instance, which results in a sudden drop in the torque or driving force of the engine, and on the other hand, when the driving wheel leaves the excessive slip state, fuel supply is suddenly resumed, resulting in a sudden increase in the torque or driving force of the engine. Thus, according to the proposed control system, the driver or passenger will feel a shock, i.e. degraded driveability both at the time of transition to the excessive slip state and at the time of transition from the excessive slip state.
A solution to this problem would be to reduce the fuel supply to the engine instead of completely cutting off the fuel supply as above, so as to progressively reduce the torque of the engine. However, the slip rate variation of the driving wheels varies depending upon running conditions of the vehicle. Therefore, if the vehicle is in a running condition wherein the slip rate variation is large, mere decrease in the torque of the engine according to the solution would lead to insufficient control of the slip so that a desired slip rate will be attained with much delay, resulting in degraded driveability.
While internal combustion engines in general have combustion characteristics varying with engine rotational speed, the above proposed control system does not contemplate slip control in accordance with engine rotational speed. Therefore, it would fail to properly control the combustion characteristics when the driving wheels have shifted to an excessive slip state. A result of this failure would be emission of unburnt fuel in large quantities, leading to so-called after-fire, i.e. combustion of unburnt fuel in the exhaust system of the engine. Further, if an exhaust purging catalyst device, particularly a three-way catalytic converter, is provided in the exhaust system, combustion of unburnt fuel in the exhaust system will cause an excessive rise in the temperature of the catalyst device, deteriorating the performance thereof.
Furthermore, according to the proposed control system no contemplation is made of load on the engine in effecting the fuel supply control for slip control. For instance, in the case of controlling the air-fuel ratio to a leaner value in an excessive slip state, the amount of drop in the torque or driving force of an internal combustion engine to be caused by reducing the fuel amount supplied to the engine varies depending upon the magnitude of load on the engine. Therefore, in effecting slip control independently of load on the engine in the excessive slip state, if the target air-fuel ratio is set at a value conforming to a high load operating condition of the engine, slip control is effected to an excessive extent when the engine is operating in a low load operating condition, whereas if the target air-fuel ratio is set at a value conforming to a low load operating condition of the engine, slip control is effected to an insufficient extent when the engine is operating in a high load operating condition. This, therefore, makes it impossible to achieve desired values of slip rate over the entire load range, thereby failing to secure good driveability of the vehicle.
Another driving wheel slip control system is conventionally known e.g. by Japanese Provisional Patent Publication (Kokai) No. 58-8436, in which the degree of the slip is detected based on the difference .DELTA.V between the driving wheel speed and the trailing wheel speed of the vehicle, and when an excessive slip state in which the wheel speed difference .DELTA.V is great is detected, the amount of fuel supplied to the engine is decreased stepwise in the following order of (a) to (f) as the wheel speed difference .DELTA.V is increased, whereby the vibration of a vehicle caused by a sudden drop in the output torque of the engine is prevented.
(a) An air-fuel mixture supplied to a first cylinder is leaned (the amount of the mixture is reduced).
(b) Supply of fuel to the first cylinder is cut off (fuel cut to the first cylinder is executed).
(c) While supply of fuel to the first cylinder is cut off, an air-fuel mixture supplied to a second cylinder is leaned.
(d) Supply of fuel to the first and second cylinders is cut off.
(e) While supply of fuel to the first and second cylinders is cut off, an air-fuel mixture supplied to a third cylinder is leaned.
(f) Supply of fuel to the first, second and third cylinders is cut off.
An excessive slip state of a driving wheel can take place instantly as a sudden excessive slip, particularly on a road surface having a small friction coefficient (e.g. on a wet road surface). Therefore, according to the conventional control system described above, in such event the control is executed in such a manner that the control stage is directly shifted from (a) to (d), or (b) to (f).
In the meanwhile, in a type of internal combustion engine in which fuel is injected into an intake pipe thereof, fuel remains within the intake pipe in the form of fuel adhering to the intake valve and its vicinity. The amount of the residual fuel depends on the amount of fuel injected last time, and therefore it is increased particularly during acceleration of the vehicle at which the excessive slip may occur. Therefore, in the conventional system, if the control stage is directly shifted from (a) to (d), the second cylinder undergoes fuel cut immediately from the ordinary fuel injection state, i.e. the state in which there remains a large amount of fuel within the intake pipe. Accordingly, although most of the residual fuel is then drawn into the cylinder, the mixture supplied into the cylinder does not have so rich an air-fuel ratio as to cause complete burning of the fuel in the cylinder, which results in fuel being discharged into the air as unburned ingredients, whereby the exhaust emissions are degraded. Further, in the case of the control stage directly shifting from (b) to (f), for example, the above-described problem arises in a more severe form.
Further, in an automotive vehicle with an engine equipped with an exhaust gas-purifying device in an exhaust system thereof, such as a catalytic converter, the unburned ingredients will burn within the catalytic converter, thereby reducing its performance and actually damaging it.
Further, according to the conventional control system, at the control stage of (d), fuel cut to the first and second cylinders is executed, whereas an ordinary amount of fuel is supplied to the other cylinder, so that there occurs a great difference in output between the cylinders, which leads to increased pulsation of the engine torque.